Edge Removal Of Silicon-On-Insulator Transfer Wafer

ABSTRACT

A silicon-on-insulator transfer wafer having a front surface with a circumferential lip around a circular recess is polished. In one version, the circular recess on the front surface of the wafer is masked by filling the recess with spin-on-glass. The front surface of the wafer is exposed to an etchant to preferentially etch away the circumferential lip, while the circular recess is masked by the spin-on-glass. The spin-on glass is removed, and the front surface of the transfer wafer is polished. Other methods of removing the circumferential lip include applying a higher pressure to the circumferential lip in a polishing process, and directing a pressurized fluid jet at the base of the circumferential lip.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/998,289, filed Nov. 26, 2004, entitled “Edge removal ofsilicon-on-insulator transfer wafer,” the content of which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Embodiments of the present invention relate to polishing andrefurbishing a silicon-on-insulator transfer wafer.

In the fabrication of semiconductor devices, substrates are processed byforming or etching material on the substrates. An example of a substrateprocess is a process to form a silicon-on-insulator (SOI) structure on aproduction substrate, the SOI structure comprising a silicon layeroverlying an insulator layer, which is typically an oxide layer. In oneversion of a SOI formation process, an oxide layer is formed on thesurface of a first silicon wafer, and the surface is pressed against andbonded to a second silicon wafer surface. The bonded wafers can be heattreated to strengthen the bond attachment. The exposed surface of one ormore of the wafers is then ground away to leave a thin silicon layeroverlying an oxide layer on a silicon substrate. However, this processis often undesirably inefficient, as mechanical grinding or etching ofthe wafer can take an unsuitably long time. The process can also becostly, as a substantial amount of silicon material is wasted in thegrinding process, and a fresh silicon substrate is typically requiredfor each new process.

In an improved SOI transfer process, a SOI structure is transferred ontoa production wafer from a transfer wafer by bonding the transfer waferand production wafer together, and then de-laminating the transfer waferfrom the production wafer to form the SOI structure. The SOI transferwafer can comprise a silicon material such as a silicon wafer or siliconlayer, which is implanted with ions, and an overlying thermally grownsilicon oxide layer. In one version, the transfer wafer comprises asurface layer of epitaxially grown silicon which is implanted with ions,and covered with a layer of silicon oxide. The surface of the siliconoxide layer on the transfer wafer is contacted with a surface of aproduction wafer, which is typically a bare silicon wafer, and thewafers are adhered to one another. A subsequent de-lamination step isperformed to cleave or de-laminate the transfer wafer from theproduction wafer at the ion-implanted layer, leaving the oxide layer anda portion of the remaining silicon material bonded to the productionwafer to provide the SOI structure.

The de-laminated transfer wafer typically has a remaining portion of thesilicon material, on which an oxide layer can be re-formed to performadditional SOI transfer processes. However, because the periphery of thetransfer and production wafer are typically not bonded as strongly asthe central region, a circumferential lip often forms about theperiphery of the transfer wafer after the delamination process. Thecircumferential lip comprises un-transferred portions of the siliconmaterial, as well as residual oxide insulator material. Such acircumferential lip is undesirable because it impedes surface contactbetween the transfer and production wafers, and limits the ability ofthe transfer wafer to be re-used for subsequent SOI processes.Accordingly, it is desirable to have a method of refurbishing a transferwafer to permit transfer of SOI layers onto a plurality of subsequentwafers.

In one version, the transfer wafer is refurbished by performing adouble-sided polishing step, such as by chemically mechanicallypolishing the wafer, to remove remaining oxide material from both sidesof the wafer and polish away the circumferential lip. In yet anotherversion described in U.S. Pat. No. 6,284,628 to Kuwahara et al, issuedon Sep. 4, 2001 and assigned to Shin-Etsu Handotal Co., Ltd., which isherein incorporated by reference in its entirety, an overlying oxidelayer is removed by a chemical etching step. Afterwards, the smallerstep remaining on the wafer is removed by polishing the surface of thewafer just slightly, by inserting the wafer between polishing turntables.

However, such double-sided polishing methods can be inefficient, oftenfail to remove all of the circumferential lip, and can require anundesirably long polishing time, for example, a double-sided polishprocess may only remove about ½ a micron/minute of material. Theprocedures can also be expensive due to the quantity of slurry and otherconsumable chemical mechanical polishing pads required. Also,conventional polishing procedures may remove an undesirably large amountof the silicon layer from the transfer wafer to remove thecircumferential lip, reducing the number of times the transfer substratecan be used. Conversely, if the transfer wafer is underpolished, a smallcircumferential bumper ring, typically having a height of around 200 nm,can remain on the wafer. The remaining bumper lip can impede contactbetween the transfer wafer and a production wafer, and reduce thequality of the SOI transfer process. For example, for recycled transferwafers having a circumferential bumper with a height of about 50 nm, theSOI transfer yield may be reduced by about 20% over fresh transferwafers having planar surfaces. Overpolishing of the circumferential lipwith respect to the rest of the wafer can result in edge roll-off wafersurface topography, which also reduces SOI transfer yields.

Accordingly, it is desirable to recycle and reclaim a SOI transfer waferto re-use the transfer substrate. It is furthermore desirable to reclaima SOI transfer wafer to provide a transfer wafer that is substantiallyabsent a circumferential lip, to provide a substantially planar contactsurface.

SUMMARY OF THE INVENTION

A method of polishing a silicon-on-insulator transfer wafer having afront surface with a circumferential lip around a circular recess,involves masking the circular recess on the front surface of the waferby filling the recess with spin-on-glass. The front surface of the waferis exposed to an etchant to preferentially etch away the circumferentiallip, as the circular recess is masked by the spin-on-glass. Thespin-on-glass is removed, and the front surface of the transfer wafer isfurther polished.

In another version, the front surface of a silicon-on-insulator transferwafer is ground with particulates having a mesh size of from about 2000to about 8000, and then etched. The back surface of the transfer waferis etched to remove the oxide layer, and the front surface of the waferis further polished.

In yet another version, the front surface of a silicon-on-insulatortransfer wafer is exposed to an etchant solution having a compositioncapable of preferentially etching away the implanted circumferential liprelative to etching at the circular recess. Following exposure to theetchant solution, the front surface of the transfer wafer is polished.

In a further version, the circumferential lip on the front surface ofthe transfer wafer is ground off substantially without grinding thecircular recess. The front surface of the transfer wafer is thenpolished.

In yet another version, the transfer wafer is rotated against apolishing pad having an edge while a polishing slurry is applied betweenthe transfer wafer and polishing pad. The edge of the polishing pad ismaintained at a higher pressure against the circumferential lip of thetransfer wafer relative to a pressure maintained at the circular recess,so that the pad preferentially polishes off the circumferential lip ofthe transfer wafer.

In still another method, a fluid jet is directed at an angle toward abase of the circumferential lip of the transfer wafer, to etch away thecircumferential lip. The front surface of the transfer wafer is thenpolished.

BRIEF DESCRIPTION OF THE DRAWINGS

These features, aspects and advantages of the present invention willbecome better understood with regard to the following description,appended claims, and accompanying drawings, which illustrate examples ofthe invention. However, it is to be understood that each of the featurescan be used in the invention in general, not merely in the context ofthe particular drawings, and the invention includes any combination ofthese features, where:

FIG. 1 a is a sectional side view of an embodiment of a SOI transferwafer having an insulator layer over a silicon material;

FIG. 1 b is a sectional side view of the transfer wafer of FIG. 1 a,attached to a production wafer during a SOI transfer process;

FIG. 1 c is a sectional side view of the transfer and production wafersof FIG. 1 b after de-lamination of the transfer wafer from theproduction wafer, the de-laminated transfer wafer having acircumferential lip, and the production wafer having a SOI structure;

FIG. 2 a is a sectional side view of an embodiment of a transfer waferhaving a circumferential lip and a circular recess at least partiallycovered by a mask layer;

FIG. 2 b is a sectional side view of the transfer wafer of FIG. 2 aafter removal of the circumferential lip;

FIG. 2 c is a sectional side view of the transfer wafer of FIG. 2 bafter removal of the mask layer;

FIG. 2 d is a sectional side view of the transfer wafer of FIG. 2 cafter removal of an insulator layer;

FIG. 3 is a sectional side view of an embodiment of a transfer waferhaving a circumferential lip, and a grinding wheel comprising finegrinding particulates for grinding the circumferential lip;

FIG. 4 is a sectional side view of an embodiment of a transfer waferhaving a circumferential lip, and a grinding tool capable of selectivelygrinding the circumferential lip;

FIG. 5 is a sectional side view of an embodiment of a transfer waferhaving a circumferential lip, and a polishing pad capable of maintaininga higher pressure at an edge of the pad to grind away thecircumferential lip;

FIG. 6 is a sectional side view of an embodiment of a transfer waferhaving a circumferential lip, and a fluid jet nozzle capable ofdirecting a fluid jet stream at a base of the circumferential lip toetch away the circumferential lip;

FIG. 7 a is a perspective view of a chemical mechanical polisher capableof polishing a transfer wafer;

FIG. 7 b is a partially exploded perspective view of the chemicalmechanical polisher of FIG. 7 a;

FIG. 8 a is a flow chart illustrating an embodiment of a refurbishmentprocess having a double-sided polishing step; and

FIG. 8 b is a flow chart illustrating an embodiment of a refurbishmentprocess with a single-sided polishing step.

DETAILED DESCRIPTION OF THE INVENTION

An example of a transfer wafer 102 suitable for use in a layer transferprocess, such as a silicon-on-insulator (SOI) transfer process, is shownin FIG. 1 a. For a SOI transfer process, the transfer wafer 102comprises a silicon material 105, such as for example a silicon wafer103. The transfer wafer 102 may also comprise a silicon material 105comprising a surface layer 104 of silicon, such as surface layer 104 ofsilicon that is epitaxially grown on the silicon wafer 103, as shown forexample in FIG. 1 a. An insulator layer 106 is formed over a surface 107of the silicon material 105. For example, the insulator layer 106 may beformed over an epitaxially grown silicon layer 104, as shown for examplein FIG. 1 a, or may alternatively be formed directly over a surface 107of a bare silicon wafer 103, as shown for example in FIG. 2 a. Theinsulator layer 106 can comprise silicon oxide, and may also compriseother insulator materials such as at least one of Al₂O₃, AlN, SiC, andSi₃N₄. An ion-implanted layer 108 is formed at a desired depth d in thesilicon material 105 by ion-implanting hydrogen or other ions, such asoxygen ions, into the silicon material 105. The ion-implanted layer 108may also be sloped at the periphery of the wafer 102 due to beveling ofthe wafer 102, as shown for example in FIG. 2 a. While the presentembodiment is described in terms of a transfer wafer 102 having asilicon material 105 below an insulator layer 106, it should beunderstood that it may be desirable to similarly transfer other layerstructures, such as for example a germanium-on-insulator structure, andthus the invention should not be limited to the transfer ofsilicon-on-insulator structures only.

The SOI transfer process typically comprises contacting a front surface114 of the transfer wafer 102 with a surface 116 of a production wafer110, as shown for example in FIG. 1 b. The production wafer 110 can be asilicon wafer, such as for example a polished bare silicon wafer, or maycomprise other materials such as germanium. In one version, an insulatorlayer such as a silicon oxide layer may be formed at the surface 116 ofthe production wafer 110 (not shown), either instead of or in additionto the insulator layer 106 on the transfer wafer 102. The contactingwafer surfaces 114, 116 bond together to form a silicon-on-insulatorstructure 120 comprising a layer of the silicon material 105 over theinsulator layer 106 on the production wafer 110.

The transfer wafer 102 is delaminated from the production wafer 110 toform the silicon-on-insulator structure 120 remaining on the productionwafer 110, as shown for example in FIG. 1 c. The transfer wafer 102 canbe de-laminated or cleaved from the production wafer 110 by activatingthe ion implanted layer 108, which delaminates the portion of thetransfer wafer 102 above the layer 108. In one version, the ionimplanted layer 108 is activated by heating the bonded transfer andproduction wafers 102, 110 to a temperature of at least about 1000° C.,which alters the crystal structure at the ion-implanted layer 108 tode-laminate the transfer wafer 102 from the production wafer 110 at theion-implanted layer 108. Once the transfer wafer 102 has been removed,the remaining silicon-on-insulator structure 102 comprising thetransferred silicon material 105 and insulator layer 106 can be furthertreated, for example by heat treating and polishing, as described forexample in U.S. Pat. No. 6,284,628 to Kuwahara et al, published on Sep.4, 2001 and assigned to Shin-Etsu Handotal Co., Ltd., which is hereinincorporated by reference in its entirety.

The de-laminated transfer wafer 102 typically comprises the remainingsilicon material 105 that lay previously behind the ion-implanted layer108. Furthermore, as the edges 118 of the transfer wafer 102 typicallydo not bond strongly to the production wafer 110, the de-laminatedtransfer wafer 102 also typically comprises an attached circumferentiallip 112 about a circular recess 122 comprising remaining siliconmaterial 105. The circumferential lip 112 can comprise the peripheralinsulator material 106 and silicon material 105 above the ion-implantedlayer 108 that remain bonded to the transfer wafer 112. For example, thesilicon step 126 can comprise remaining, non-transferred silicon wafermaterial, as shown for example in FIG. 2 a. Alternatively, the siliconstep 126 can comprise silicon material remaining from an epitaxiallygrown silicon layer 104 formed over a silicon wafer 103, as shown forexample in FIG. 1 c. The circumferential lip 112 can comprise a siliconstep 126 having a thickness above the surface 114 of the wafer 102 offrom about 10 nanometers to about 500 nanometers. The circumferentiallip 112 can also comprise an insulator portion 124 on top of and even tothe side of the silicon step 126, as shown for example in FIGS. 1 c and2 a, and may comprise a thickness of from about 10 nanometers to about200 nanometers. A width of the circumferential lip 112 may be from about1 millimeter to about 5 millimeters. The circumferential lip 112 mayalso comprise a beveled shape, as shown for example in FIG. 2 a, withsurface of the lip 112 sloping downwardly at the periphery of the wafer102, which may be the result of beveling of the wafer before the SOItransfer process.

The circumferential lip 112 can interfere with re-use of the transferwafer 102 for subsequent SOI transfer processes, because the lip 112 caninhibit smooth and even contact between the transfer wafer andproduction wafer surfaces. Also, the front surface 114 of the transferwafer 102 can comprise damaged or rough regions that requirerefurbishment. Accordingly, it is desirable to refurbish the transferwafer 102 and remove the circumferential lip 112 to reclaim and preparethe transfer wafer 102 for re-use in subsequent SOI transfer processes.

Also, the transfer wafer 102 may comprise remaining side and back layers128, 130 of insulator material, which may be desirable to remove beforere-use of the transfer wafer 102. For example, in some embodiments, thetransfer wafer 102 may be exposed to an etchant solution to remove oneor more of the side and back layers 128, 130 of insulator material.Alternatively, one or more of the side and back layers 128, 130 ofinsulator material may be retained on the transfer wafer 102, andremoved once removal of the circumferential lip 112 has been completed.An etchant solution capable of removing the insulator material may be anacidic solution, such as a solution of HF, having a concentration of,for example, from about 0.5% to about 10% by volume.

Masking Method

In one version, a method of removing the circumferential lip 112comprises masking the circular recess 122 before removing the exposedcircumferential lip 112. In FIG. 2 a, the transfer wafer 102 issubstantially absent an epitaxially grown silicon layer 104, and thusthe circumferential lip 112 has a silicon step 126 comprisingnon-transferred silicon wafer material originating from a bare siliconwafer 103. In an alternative version, the silicon step 126 may compriseremaining material from an epitaxially grown silicon layer 104, as shownfor example in FIG. 1 c. In one version of the method, the circularrecess 122 can be at least partially filled with a mask 133 comprising asuitable masking material. The mask 133 desirably comprises an etchresistant material, such as for example spin-on glass, which can bereadily applied to the circular recess 122 and exhibits good resistanceto etching solutions. Etch resistant materials can include, for example,Accuglass™ commercially available from Honeywell Electronic Materials,and Fox™ Flowable Oxide, commercially available from Dow CorningCorporation. In one version, a spin-on glass mask 133 can be formed byspin-coating a solution comprising silicon oxide suspended in a solventonto the circular recess 122. The spin-on glass mask 133 can be annealedto planarize the mask 133, by heating to a temperature of at least about700° C., and even at least about 900° C. Providing a planarized mask 133promotes even masking and protection of the material underneath themasking layer.

Once the mask 133 has been formed, the circumferential lip 112 can beexposed to an etchant that preferentially etches away thecircumferential lip 112 substantially without excessively etching themask 133, to substantially remove the circumferential lip 112 from thetransfer wafer 102, as shown for example in FIG. 2 b. An example of asuitable etchant for preferentially etching the circumferential lip 112may be a chemical solution comprising a base, such as a solution of KOH.Other examples of suitable chemical etchants may include, for example,at least one of NH₄OH, and tetramethyl ammonium hydroxide (TMAH). In oneversion, the circumferential lip 112 is removed by immersing the frontface 114 of the transfer wafer 102 in a solution comprising KOH in aconcentration of from about 10% to about 60% by weight, such as about40% by weight, while maintaining the temperature of the solution atleast about 50° C., such as from about 50° C. to about 80° C., and evenat least about 90° C. The chemical etchant removes the circumferentiallip 112 by etching the exposed lip at a rate that is faster than that atwhich the mask 133 is etched. Also, as the ion-implanted layer 108extends towards the periphery of the transfer wafer 102, thecircumferential lip 112 will generally be relatively loosely held on thetransfer wafer 102, rendering it more readily eroded and removed thanthe masked regions. The circumferential lip 112 is exposed to theetching solution for a duration sufficient to substantially remove thecircumferential lip 112, such as for about 30 seconds to about 5minutes.

Once the circumferential lip 112 has been sufficiently etched, the mask133 can be removed from the transfer wafer 102 to reveal the underlyingsilicon material 105, as shown for example in FIG. 2 c. The mask 133 canbe removed by exposing the front face 114 of the transfer wafer 102 to achemical etchant that is capable of etching away the masking material,preferably at a rate that is higher than the rate at which the exposedperipheral region 135 of the transfer wafer 102 is etched. A suitableetchant may comprise an acidic solution, such as a solution of HF. Othersuitable etchants may comprise, for example, at least one of a bufferedoxide etch solution (BOE) comprising a solution of HF buffered withNH₄F, and a mixed etch solution (MAE), which can comprise a mixture ofHF, HNO₃, and acetic acid. In one version, a mask 133 comprising spin-onglass is removed by immersing the front face 114 of the transfer wafer102 in an etchant solution comprising HF at a concentration of fromabout 3% to about 10% by volume for a duration of from about 2 to about10 minutes. The mask removal step provides a transfer wafer frontsurface 114 that is substantially absent a circumferential lip 112, asshown for example in FIG. 2 c. The chemical etchant may also be capableof removing one or more of a remaining side and backside layer 128, 130of insulator material along with the mask 133, to provide a wafer 102that is substantially absent the insulator material, as shown forexample in FIG. 2 d.

Selective Etching Method

In yet another version, the circumferential lip 112 is removed byexposing the front surface 114 of the transfer wafer 102 to an etchantsolution comprising a composition that is capable of preferentiallyetching away the circumferential lip 112 relative to the material in thecircular recess 122. For example, the etchant solution may be capable ofetching the ion-implanted circumferential lip 112 at a faster rate thanthe silicon material in the circular recess 122. The ion-implanted layerregions 108 on the periphery of the transfer wafer 102 typically havelattice damage, which can render the circumferential lip 112 over thecleaning layer 108 easier to remove with certain etchant compositionsthan other regions of the wafer 102. The circumferential lip 112 can beetched with a selected etchant composition until the lip 112 has beensufficiently removed, for example to provide a transfer wafer 102 havinga substantially planar front surface 114, as shown in FIG. 2 c.

A suitable chemical etchant for removing the circumferential lip 112 maybe a basic etchant solution, such as KOH. Suitable chemical etchants mayalso comprise at least one of tetramethyl ammonium hydroxide (TMAH) andethylenediamine (EDA). Desirably, the etchant solution has a baseconcentration and composition that is selected to preferentially etchaway an ion-implanted circumferential lip 112 having an ion implanteddepth of from about 100 nm to about 2 micrometers, relative to acircular recess 122 that is substantially absent ion-implantation. Forexample, the etchant solution may be capable of etching theion-implanted circumferential lip 112 at a rate that is at least about1.5 times to about 3 times the rate at which the circular recess 122 isetched. In one version, the circumferential lip 112 can be removed byexposing the front surface 114 of the transfer wafer 102 to an etchantsolution comprising KOH in a concentration of from about 10% to about60% by weight, that may be maintained at a temperature of at least about40° C., such as from about 40° C. to about 80° C., and even at leastabout 90° C., for a duration of from about 30 seconds to about 3minutes.

In one version, the front surface 114 of the transfer wafer 102 may beexposed to a solution having a composition that is selected to remove aninsulator portion 124 from the top of the circumferential lip 112 touncover a remaining silicon step portion 126 of the circumferential lip112. For example, the front surface 114 may be immersed in a solutionthat selectively etches the insulator material on the front surface 114at a higher rate with respect to the silicon material on the surface,such as for example silicon material in the circular recess 122. Asuitable selective etchant may be for example an oxide removal etchant,such as for example a solution of HF in a concentration of from about 3%to about 10% by volume. In one version, a suitable wet etching tool foretching the front surface of the wafer 102 may be an Oasis wet etchingtool commercially available from Applied Materials. A back layer 130 ofinsulator material may be retained during this etching step, for exampleby keeping the back surface of the transfer wafer 102 out of the etchingsolution that removes the front side insulator material, such that theback layer 130 is substantially not etched. A thickness of the retainedbackside insulator layer 130 may be from about 100 nm to about 300 nm.Once the insulator material has been removed from the front surface toreveal the silicon step portion 126 of the circumferential lip 112, thetransfer wafer 102 may be immersed an etching solution that selectivelyetches the ion-implanted regions of the wafer 102, such as theion-implanted silicon step portion 126 of the circumferential lip. Theion-implanted region selective etching solution can comprise at leastone of the selective etching solutions described above, such as forexample a solution of KOH in a concentration of from about 10% to about60% by weight. The remaining back layer 130 of insulator materialinhibits excessive etching of the back side of the transfer wafer 102,while the ion-implanted circumferential lip 112 is etched and removed bythe selective etching solution. Thus, this process may extend the usablelife of the transfer wafer 102 by reducing excessive etching of the backside of the transfer wafer 102.

Fine Particulate Grinding Method

In yet another version, the circumferential lip 112 can be removed bygrinding the front surface 114 of the wafer 102 with relatively fineparticulates. The relatively fine particulates may be capable ofremoving the circumferential lip 112 substantially without damaging thefront surface 114 or underlying lattice structure of the transfer wafer102. A suitable size of the particulates may be a mesh size of fromabout 2000 to about 8000, which corresponds to an average particle sizeof no larger than about 10 microns. The relatively fine particulates maybe ground against the front surface 114 until the circumferential lip112 has been substantially removed from the front surface 114 of thetransfer wafer 102.

In one version, the front surface 114 can be ground with a grindingwheel 132 having the relatively fine particulates fixed thereon, asshown for example in FIG. 3. The particulates may be on a grindingsurface 134 that is pressed and rotated against the front surface 114,and in particular against the circumferential lip 112, to grind away andremove the circumferential lip 112. A suitable grinding wheel 132 maycomprise for example fine particles of at least one of diamond, cubicboron nitride or other abrasive material, that is fixed onto thegrinding surface 134. For example, the grinding wheel 132 can comprisefine abrasive particles that are suspended in at least one of a resinand vitrified matrix on the grinding surface 134, and may even besuspended by metal bonds on the grinding surface 134, such as forexample fine grinding wheels commercially available from Saint Gobain,and Asahi Glass Co, Ltd. A polishing slurry can also be applied betweenthe circumferential lip 112 and grinding wheel 132 to improve theremoval of the lip, such as for example a slurry comprising particles ofsilica in a solution comprising at least one of KOH and tetramethylammonium hydroxide.

The transfer wafer 102 can be held on a support 136, such as a vacuumchuck, during the grinding process. The support 136 may hold thetransfer wafer 102 substantially stationary while the grinding wheel 132is rotated against the front surface 114. Alternatively, the support 136may rotate the transfer wafer 102 against a stationary grinding surface134, or the grinding wheel 132 and transfer wafer 102 may both berotated, for example in opposing directions. In one version, thetransfer wafer 102 is kept stationary, and a grinding wheel 132comprising the relatively fine particles is rotated while pressingagainst the front surface 114 at a rate of from about 300 rpm to about900 rpm.

It may furthermore be desirable to retain the backside layer 130 ofinsulator material during the grinding process, to inhibit contaminationof the transfer wafer 102 by contaminant materials from the support 136.The backside layer 130 can provide a barrier layer between the support136 and the silicon portions of the transfer wafer 102 to inhibit thediffusion of contaminant particles, such as metal support materialparticles, into the silicon portions of the transfer wafer 102. Thebackside layer 130 can also protect the wafer 102 from microscratchesthat can results from abrasion of the wafer 102 against the support 136.Desirably, once the backside insulator layer 130 has been removed, forexample by immersing in an etching solution, the resulting back surface138 of the transfer wafer 102 has a metal contamination level of lessthan about 1×10¹² atoms/cm², such as less than about 5×10¹⁰ atoms/cm².At least one of the backside layer 130 and other portions of insulatormaterial may be removed in a post-etching process comprising a solutionof, for example, hydrofluoric acid.

Selective Grinding Method

In yet another method of refurbishing the transfer wafer 102, thecircumferential lip 112 can be removed by a selective grinding processthat grinds off the circumferential lip 112 on the front surface 114 ofthe transfer wafer substantially without grinding the circular recess122. This method allows the circumferential lip 112 to be selected forand removed, substantially without damaging or eroding away the materialin the circular recess 122. The method can provide a substantiallyplanar front surface 114 suitable for use in subsequent SOI transferprocesses.

FIG. 4 shows an example of a selective grinding process with a grindingtool 142 that is capable of grinding off the circumferential lip 112substantially without grinding the circular recess 122. The grindingtool 142 comprises a roughened surface 144 that grinds against thecircumferential lip 112 to wear away the lip. For example, the grindingtool 142 can comprise ridges or other raised particles or features on acylindrical grinding head 146 that grind against the circumferential lip112. In one version, the grinding tool 142 can even comprise relativelyfine particulates that are capable of relatively gently grinding thecircumferential lip 112, such as particulates having a size of fromabout 2 micrometers to about 6 micrometers. The grinding tool 142 can beshaped and sized to selectively grind the circumferential lip 112 of thetransfer wafer 102, substantially without grinding the circular recess122. For example, the grinding tool 142 may comprise a grinding head 146that is adapted to rotate about a circumference of the transfer wafer102, and may comprise a grinding surface 144 having a width that issized to extend across substantially only the circumferential lip 112.

In one version, the grinding tool 142 comprises a grinding surface 144that is an abrasive tape 145 having abrasive particles thereon. Theabrasive tape 145 can be continuously replaced on the grinding head 146by feeding fresh tape 145 onto the grinding head 146 to maintaingrinding performance. The abrasive tape 145 may also comprise a widthacross the grinding head 146 that is sized to accommodate thecircumferential lip 112, substantially without grinding the circularrecess 122. The grinding tool 142 may also comprise, for example,detection or positioning software that is adapted to determine alocation of the grinding head 146 on the wafer 102 to position thegrinding head 146 over the circumferential lip 112. A suitable grindingtool may comprise, for example, a 7AF nTellect grinding machinecommercially available from Strasbaugh, or a NME-68 Polishing Machinecommercially available from Mipox International Inc. The grinding head146 is desirably rotated at a sufficient velocity to wear away thecircumferential lip 112, substantially without inducing microcracking orother damage in the circular recess 122 on the transfer wafer 102. Forexample, a suitable rotational velocity of the grinding head 146 may beless than about 3500 rpm, such as from about 2500 to about 3500 rpm.

In one version, the transfer wafer 102 can be held by a support 136during the selective grinding process. A backside insulator layer 130may be kept on the transfer wafer 102 during the grinding process toinhibit contamination of the transfer wafer 102 from the substratesupport 136. The support 136 may be a rotatable support 136 that iscapable of revolving the transfer wafer 102 below the grinding tool 142so that the circumferential lip 112 is below the grinding tool 142. Inanother version, the transfer wafer 102 may be held on a non-rotatablesupport, and the grinding tool 142 may revolve about the periphery ofthe transfer wafer 102 to selectively remove the peripheral lip. Thegrinding tool 142 and support 136 may also be capable of simultaneouslyrevolving in the same or opposing directions during the grindingprocess, to provide the desired grinding characteristics.

Selective CMP Method

In yet another method, the circumferential lip 112 may be removed by animproved chemical mechanical polishing method that comprises rotatingthe front surface 114 of the transfer wafer 102 against a polishing pad152, while applying a higher pressure to a peripheral edge 150 of thepad 152 above the circumferential lip 112 of the transfer wafer 102,relative to a pressure maintained at a central pad region 154 above thecircular recess 122, as shown for example in FIG. 5. The differential inpressure allows for the circumferential lip 112 to be preferentiallypolished from the transfer wafer 102 at a higher rate, substantiallywithout excessively polishing the circular recess 122. For example, thepressure applied to the peripheral edge 150 of the polishing pad 152 maybe at least about 14 kPa (2 pounds-per-square-inch) greater, and even atleast about 34 kPa (5 PSI) greater than the pressure applied at thecentral pad region 154. In one version, the pressure applied to theperipheral edge 150 is from about 7 kPa (1 PSI) to about 83 kPa (12PSI), such as about 55 kPa (8 PSI), and the pressure applied to thecentral pad region 154 may be from about 3 kPa (0.5 PSI) to about 34 kPa(5 PSI), such as about 14 kPa (2 PSI). A suitable polishing head 153that is capable of applying different pressures to the pad 152 maycomprise the Titan Head, Titan Profiler Head, and Contour Head,commercially available from Applied Materials, Inc. A suitable pad 152may comprise a polyurethane pad.

A polishing slurry can also be applied between the transfer wafer 102and polishing pad 152 during the polishing process. The polishing slurrymay be capable of both chemically and physically eroding thecircumferential lip 112 to remove the lip from the transfer wafer 102.For example, the polishing slurry can comprise slurry particles in achemical etchant, such as at least one of KOH and NH₄OH. The slurryparticles can comprise at least one of fumed and colloidal silicon, andmay have a specific surface area of from about 50 m²/gram to about 200m²/gram. In one version, the polishing slurry is preferentially appliedbetween the peripheral edge 150 of the pad and the circumferential lip112 of the transfer wafer 102 to polish away the circumferential lip112, substantially without excessively eroding or damaging the materialin the circular recess 122. As an alternative to applying differentpressures to a polishing pad 152, in another version, the pad 152 isformed of a material that is relatively inflexible. The pad 152 may besufficiently inflexible such that when the pad 152 is pressed againstthe circumferential lip 112 of the wafer 102, the central region 154 ofthe pad 152 substantially does not bend or deform into the circularrecess 122 of the wafer 102, and thus grinding and removal of materialfrom the recess 122 is substantially inhibited. The relativelyinflexible and hard polishing pad 152 thus provides for selectiveremoval of the circumferential lip 112 substantially without grinding orremoving other portions of the wafer 102. A suitable polishing pad maycomprise, for example, a Rodel™ MH polishing pad or a Rodel Suba™ 1200,both of which are commercially available from Rodel, U.S.A.

During the selective polishing process, the transfer wafer 102 can beheld on a support 136 that is capable of rotating the wafer 102 againstthe polishing pad 152. Alternatively, the support 136 may benon-rotatable, and the polishing pad 152 may rotate to polish the wafer120, or the support 136 and pad 152 can both be rotatable. Desirably,the backside insulator layer 130 is retained during the polishingprocess to inhibit contamination of the wafer 102 by contaminantmaterials from the wafer support 136.

Fluid Jet Spraying Method

In yet another method, the circumferential lip 112 can be removed bydirecting a fluid jet 156 at a base 158 of the circumferential lip 112to etch away the circumferential lip 112. The fluid jet 156 cancomprise, for example, a jet of pressurized water that is aimed at thebase 158 of the circumferential lip 112, in the area of the ionimplanted layer 108, to loosen portions of the circumferential lip 112from the transfer wafer 102. The fluid jet 156 can widen and propagatemicrocracks and voids in the layer 108, to break off the circumferentiallip 112 from the transfer wafer 102. The fluid jet 156 comprises apressure that is sufficiently high to remove the circumferential lip112, such as for example a pressure of from about 690 kPa (100 PSI) toabout 13790 KPa (2000 PSI), such as at about 1379 kPa (200 PSI). Thefluid jet 156 can be directed towards the transfer wafer 102 at an angleof from about 5° to about 40° with respect to a normal 170 to the frontsurface 114, such as at an angle of about 20°, to remove thecircumferential lip 112.

In one version, one or more of the fluid jet 156 and the transfer wafer102 may be rotated while the fluid jet 156 is directed towards the base158 of the circumferential lip 112. The rotation allows thecircumferential lip 112 to be loosened from the transfer wafer 102 onall sides, improving the overall lip removal efficiency. The fluid jet156 can be rotated, for example, by a robot arm (not shown) that iscapable of rotating a fluid jet nozzle 160 in a circular path over thetransfer wafer 102. The transfer wafer 110 can be rotated by placing thewafer 102 on a rotatable support 136. In this version, the transferwafer 102 desirably retains a backside layer 130 of insulator materialto inhibit contamination of the transfer wafer 102. A suitable rotationspeed for at least one of the fluid jet 156 and transfer wafer 102 canbe a speed of from about 10 rpm to about 500 rpm, such as about 100 rpm.The fluid jet removal method desirably removes substantially the entirecircumferential lip 112, substantially without excessively etching ordamaging the circular recess 122.

Once the circumferential lip 112 has been removed from the transferwafer 102 by any of the processes, one or more subsequent chemicalmechanical polishing processes can be performed to polish and planarizeat least one of the front surface 114 and back surface 138 of thetransfer wafer 102. The chemical mechanical polishing process can alsoremove any remaining portions of the back or side insulator layers 130,128, such as a backside insulator layer 130 that was retained to inhibitcontamination of a wafer 102 being held on a support 136. A suitablechemical mechanical polishing process can comprise applying a polishingslurry between a polishing pad and the desired surface of the transferwafer 102, and rotating the desired surface against a polishing pad. Forexample, the polishing slurry can comprise at least one of fumed andcolloidal silicon in a solution of at least one of KOH and NH₄OH.Additionally or alternatively, the backside and any side insulatorlayers 130, 128, can be removed by etching away the layers, for examplewith an acidic chemical solution comprising HF.

The refurbished transfer wafer 102 desirably comprises a substantiallyplanar front surface 114 that is substantially absent thecircumferential lip 112, and is even substantially absent any small edgeroll-off or edge bumper rings, which may be in the 0.1 nanometer sizerange. The transfer wafer 102 is also desirably absent metalcontamination that could affect the electrical properties of theprocessed production wafer, for example, the transfer wafer 102 maycomprise a metal concentration level of less than about 1×10¹²atoms/cm². The refurbished transfer wafer 102 can be re-processed, forexample by performing an ion-implantation step and insulator layerformation step, to form an SOI structure that can be transferred to aproduction wafer 110, as in FIGS. 1 a through 1 c. The improved SOItransfer wafer 102 that is refurbished to provide the substantiallyplanar front surface 114 provides improved results in the formation ofSOI structures on production wafers 110. For example a SOI transferwafer refurbished according to the described methods may be capable ofproviding at least about a 1%, and even at least about a 20% increase inthe production wafer yield.

In one version, a front surface polishing process, such as one of therefurbishing and polishing methods described above, is performed topolish the front surface 114 of the wafer 102, substantially withoutpolishing the back surface 138 of the transfer wafer 102. For example,in one version, a transfer wafer 102 that has had the circumferentiallip 112 removed is inspected by a metrology method, and the side surfaceof the wafer 102 is polished. A single side polishing step is thenperformed to polish the front surface 114 of the wafer 102,substantially without polishing the back surface 138 of the wafer. Thepolishing process used in any of the steps may be, for example, achemical mechanical polishing process. The polished wafer 102 can beinspected, subjected to further metrology, and may undergo furthercleaning processes. In contrast, some polishing processes typicallypolish both the front surface 114 and back surface 138 of the wafer 102,typically simultaneously. This double-sided polishing process in whichboth the front and back surfaces 114, 138 of the wafer 102 are polished,typically provides a relatively high removal rate of material from thewafer 102, and thus has been considered to be an efficient means ofrefurbishing the transfer wafer 102. However, such double sidedpolishing methods also typically remove an undesirably large amount ofmaterial, reducing the number of times the transfer wafer 102 can beused for SOI transfer processes. By providing a method that is capableof efficiently polishing a front surface 114 of the wafer 102substantially without requiring polishing of the back surface 138, suchas the refurbishing and polishing methods described above, the frontsurface 114 of the SOI transfer wafer 102 can be efficiently refurbishedwithout excessive loss of substrate material.

To illustrate the improved process, FIG. 8 a shows the steps of anembodiment of a refurbishment method having a double-sided polishingstep, and FIG. 8 b shows the steps of an embodiment of an improvedrefurbishment process having a single-sided polishing step. In FIG. 8 a,a SOI transfer wafer 102 is first subjected to an incoming inspectionand metrology of the wafer 102. The edge 118 of the wafer 102 ispolished to remove any excess insulator material from the edge 118, andan edge polish cleaning step is performed to clean the wafer 102 afterthe edge polish. A double-sided polishing step, such as a chemicalmechanical polishing step, is then performed to simultaneously polishthe front and back surfaces 114, 138 of the transfer wafer 102 at arelatively high rate. After a desired amount of material has beenremoved, a post-polishing cleaning step is performed to clean thetransfer wafer 102, and the wafer is inspected to determine the extentof refurbishment. A single-sided polishing step, using a conventionalpolishing method, may be performed to provide a slow removal ofremaining undesired material, followed by post cleaning, metrology andfinal cleaning steps, before shipping of the refurbished transfer wafer102.

In contrast, FIG. 8 b illustrates an efficient method of refurbishingthe transfer wafer 102 with a single-sided polishing process. In thismethod, the transfer wafer 102 is subjected to an incoming inspectionand metrology, and an edge polishing step to remove a remaining edgeinsulator layer can be performed. A single-sided polishing step isperformed to polish the front surface 114 of the wafer 102 tosubstantially remove the circumferential lip 112 from the surface 114,for example by one of the methods described above. For example, thesingle sided polishing process may comprise a chemical mechanicalpolishing method in which a higher pressure is applied by the polishingpad on the circumferential lip 112 than above the circular recess 122.As another example, the single-sided polishing process can compriseremoving the circumferential lip 112 by at least one of a selectivegrinding method, fine particulate grinding method, masking method,selective etching method, and fluid jet spraying method, optionallyfollowed by chemical mechanical or other polishing method to polish thefront surface 114. Once the front surface 114 has been polished, thetransfer wafer 102 is inspected, metrology is performed, and the waferis cleaned in a final cleaning process before shipping the transferwafer 102. Thus, the improved refurbishment and polishing methodprovides an efficient refurbishment method with fewer steps than theembodiment shown in FIG. 8 a, and also does not remove an excessiveamount of substrate material, at least in part by virtue of the absenceof a back side polishing step. The improved efficiency and reduced wasteof the method allows for the transfer wafer 102 to be more efficientlyrecycled and refurbished a greater number of times.

An embodiment of a chemical mechanical polishing apparatus 100 that issuitable for polishing one or more sides of the transfer wafer 102, isshown in FIGS. 7 a through 7 c. The apparatus 100 may be suitable forpolishing the wafer 102 after removal of the circumferential lip 112,and may also be used to polish the wafer 102 before or during the lipremoval process. Generally, the polishing apparatus 100 includes ahousing 304 containing multiple polishing stations 308, a wafer transferstation 312, and a rotatable carousel 316 that can operate independentlyrotatable wafer holders 320. A wafer loading apparatus 324 includes atub 326 that contains a liquid bath 332 in which cassettes 336containing wafers 102 are immersed, and is attached to the housing 304.For example, the tub 326 can include cleaning solution or can even be amegasonic rinsing cleaner that uses ultrasonic sound waves to clean thewafer 102 before or after polishing, and a dryer could also be provided.An arm 344 rides along a linear track 148 and supports a wrist assembly352, which includes a cassette claw 354 for moving cassettes 336 from aholding station 155 into the tub 326 and a wafer blade 356 fortransferring wafers from the tub 326 to the transfer station 312.

The carousel 316 has a support plate 360 with slots 162 through whichthe shafts 172 of the wafer holders 320 extend, as shown in FIG. 7 b.The wafer holders 320 can independently rotate and oscillateback-and-forth in the slots 162 to achieve a uniformly polished wafersurface. The wafer holders 320 are rotated by respective motors 176,which are normally hidden behind removable sidewalls 178 of the carousel316. In operation, a wafer 102 is loaded from the tub 326 to thetransfer station 312, from which the wafer is transferred to a substrateholder 320 where it is initially held by vacuum. The carousel 316 thentransfers the wafer 102 through a series of one or more polishingstations 308 and finally returns the polished wafer 102 to the transferstation 312.

Each polishing station can 308 a-c include a rotatable platen 182, whichsupports a polishing pad 184, and a pad conditioning assembly 188, asshown in FIG. 7 b. The platens 182 and pad conditioning assemblies 188are both mounted to a table top 192 inside the polishing apparatus 100.During polishing, the wafer holder 102 holds, rotates, and presses awafer 102 against a polishing pad 184 affixed to the rotating polishingplaten 182, which also has a retaining ring encircling the platen 182 toretain a wafer 102 and prevent it from sliding out during polishing ofthe wafer 102. As a wafer 102 and polishing pad 184 are rotated againsteach other, measured amounts of a polishing slurry are suppliedaccording to a selected slurry recipe, for example the polishing slurrycan comprise, deionized water with colloidal silica or alumina. Both theplaten 182 and the wafer holder 320 can be programmed to rotate atdifferent rotational speeds and directions according to a processrecipe. The polishing apparatus 100 can be used to remove a desiredamount of material from the wafer 102, such as an amount of a remaininginsulator layer, as well as to polish and planarize surfaces of thetransfer wafer 102 to render the wafer suitable for re-processing andfurther SOI transfer processes. The typical operation and generalfeatures of the polishing apparatus 100 are further described incommonly assigned U.S. Pat. No. 6,200,199 B1, filed Mar. 31, 1998 byGurusamy et al., which is hereby incorporated by reference herein in itsentirety.

The present invention has been described with reference to certainpreferred versions thereof; however, other versions are possible. Forexample, the transfer wafer 102 can comprise transferable structuresother than the SOI structure. Other embodiments and configurations ofthe CMP polisher can also be used to refurbish the transfer wafer 102.Further, alternative steps equivalent to those described for therefurbishment method can also be used in accordance with the parametersof the described implementation, as would be apparent to one of ordinaryskill. Therefore, the spirit and scope of the appended claims should notbe limited to the description of the preferred versions containedherein.

1. A method of refurbishing a silicon-on-insulator transfer wafer havinga first surface, a second surface opposing the first surface, and anedge extending between the surfaces, the method comprising: polishingthe edge of the silicon-on-insulator transfer wafer to remove excessinsulator material; simultaneously polishing the first and secondsurfaces of the silicon-on-insulator transfer wafer; and furtherpolishing the first surface of the silicon-on-insulator transfer waferto removal additional material from the first surface of the transferwafer and provide sufficient smoothness for providing a refurbishedfirst surface for the wafer.
 2. The method of claim 1 wherein thesimultaneous polishing comprises a double-sided polishing process, andthe further polishing comprises a single-sided polish process.
 3. Themethod of claim 1 which further comprises: inspecting thesilicon-on-insulator transfer wafer and performing a metrology processon the silicon-on-insulator transfer wafer prior to polishing of theedge.
 4. The method of claim 1 which further comprises cleaning thesilicon-on-insulator transfer wafer after polishing the edge and priorto the simultaneous polishing.
 5. The method of claim 1 wherein thesimultaneous polishing is conducted at a first polishing rate, thefurther polishing is conducted at a second polishing rate that is higherthan the first polishing rate.
 6. The method of claim 1 wherein thesimultaneous polishing comprises a chemical mechanical polishing step.7. The method of claim 1 which further comprises inspecting thesilicon-on-insulator transfer wafer after the further polishing toconfirm that the refurbished first surface is achieved.
 8. The method ofclaim 1 which further comprises: performing a metrology process on thesilicon-on-insulator transfer wafer and cleaning thesilicon-on-insulator transfer wafer after the further polishing.
 9. Amethod of refurbishing a silicon-on-insulator transfer wafer having afirst surface, a second surface opposing the first surface, and an edgeextending between the surfaces, the method comprising: polishing theedge of the silicon-on-insulator transfer wafer to remove excessinsulator material; simultaneously polishing the first and secondsurfaces of the silicon-on-insulator transfer wafer with a double-sidedpolishing process; and further polishing the first surface of thesilicon-on-insulator transfer wafer with a single-sided polish processto removal additional material from the first surface of the transferwafer and provide sufficient smoothness for providing a refurbishedfirst surface for the wafer; wherein the simultaneous polishing isconducted at a first polishing rate, the further polishing is conductedat a second polishing rate that is higher than the first polishing rate.10. The method of claim 9 which further comprises inspecting thesilicon-on-insulator transfer wafer and performing a metrology processon the silicon-on-insulator transfer wafer prior to polishing of theedge.
 11. The method of claim 9 which further comprises cleaning thesilicon-on-insulator transfer wafer after polishing the edge and priorto the simultaneous polishing.
 12. The method of claim 9 wherein thesimultaneous polishing comprises a chemical mechanical polishing step.13. The method of claim 9 which further comprises inspecting thesilicon-on-insulator transfer wafer after the further polishing toconfirm that the refurbished first surface is achieved.
 14. The methodof claim 9 which further comprises: performing a metrology process onthe silicon-on-insulator transfer wafer and cleaning thesilicon-on-insulator transfer wafer after the further polishing. 15.(canceled)